Many manufacturing and repair procedures rely on rotating spindles, and some require spindles that are driven by spindle units that are more or less separable from the components that support them. For example, certain milling and boring operations are performed by robotic equipment that maneuver spindle units and the rotating tools driven by such units. This equipment requires spindle units that are light in weight and highly compact, yet capable of developing high torques. In other words, the spindle units should have a high power density.
Friction stir welding (FSW) is another example of a manufacturing procedure, that relies on spindle units. This relatively new welding technique which has gained increasing popularity, is primarily used for materials, such as aluminium, that do not accommodate fusion welding well. FSW uses a cylindrical shoulder tool with profiled pin that is rotated and slowly plunged into the joint line between two workpieces of metal sheet or plate. Frictional heat that is generated at the joint softens the material of the workpieces without reaching the melting point and allows the tool to traverse the welding line, leaving a solid phase bond between the two workpieces.
Two power sources can be used to power a spindle of a spindle unit, whatever the procedure accomplished by it: a hydraulic motor or an electric motor. There are obvious advantages for using electric motors, these including ease of use, mobility and good control. The major disadvantage is, however, the power density. Typically, the electric motor is coupled directly to a tool and rotates the tool at a relatively low velocity, while delivering high torque. This requires a large and heavy motor.
One solution to improve motor power density is to increase the motor speed and use so-called gear-head motor where a gear reduction unit is integrated with an electric motor. The smaller motor operates at a higher speed but produces less torque. The gear head reduces the speed and increases the torque. There are many types of gear head motors including precision gear head motors, which are capable of running at higher speeds and generally are much more expensive than “regular” gear head motors. However, even with precision gear heads, electric motors are often limited to operating speeds of 5,000 to 6,000 rpm. This has, to a large degree, prevented the electric motors from achieving their ultimate power-density potentials.